The production of polyolefins using chromium-based catalysts is well known in the art. Various supports have been employed for such chromium-based catalysts. Silica supports have primarily been used due to their ability to form highly active polymerization catalysts. Other examples of supports that have been used for such chromium-based catalysts include alumina and aluminophosphates. Supported chromium-based catalysts were initially employed in solution polymerization processes. However, slurry polymerization soon became known as the more economical route to many commercial grades of polyolefins using such catalysts.
A polyolefin exhibits various physical, and in particular, mechanical properties that are highly affected by its molecular weight distribution (MWD). The molecular weight distribution can be determined by means of a curve obtained by gel permeation chromatography (GPC). It can be described by a parameter known as the polydispersity index (PDI), which indicates the breadth of the molecular weight distribution and is equivalent to the weight-average molecular weight of a polymer divided by the number-average molecular weight of the polymer (i.e., MW/MN). A broadening in the molecular weight distribution of a polyolefin tends to improve the flow of the polyolefin when it is being processed at high rates of shear.
The polymerization of olefins using chromium-based catalysts is often performed in the presence of hydrogen to produce polyolefins having relatively low molecular weights. However, although hydrogen can be used to regulate the molecular weight, the breadth of the molecular weight distribution of a polyolefin tends to be limited by the choice of catalyst. A need therefore exists to broaden the molecular weight distributions of polyolefins produced using chromium-based catalysts.